Mark Rules has made a bit of progress on his tiny motor controller. Someone suggested that he look at a rigid-flex board so that the driver could be wrapped around one of the motors. The idea would be to have the RS232 and MCU on one rigid board, the regulator and motor driver on a second rigid board and the connectors on a third rigid, all connected by flex. He looked at it for a while and then decided: "later."

For now, he has the placement of the four chips and all of the passives. The smallest parts were going to be 0402, but it was getting a little tight under the board and he wanted to keep the MCU bypass cap close to the power and ground. The C-delay picked for the regulator was available in an 0201 size too, so both of those were moved to 0201 packages. Without the connectors, the whole layout measures about 1/2" x 1/2".

Right now, all the passives are under the board. As much as possible, though, none are directly under the chips. Keeping them out from directly under does take up a little more space, but it allows for easier routing of the RS232 BGA and leaves room for vias in the QFN pads for the regulator and motor driver. The big PIC won't need vias, but he may use that space for a connector.

Jennifer has been tasked with building an embedded video communications system. She’s been using Arm7 LPC2138 processors in prior designs but needs quite a bit more horsepower for this project so she stepped up to the Arm11 i.MX31 from Freescale. She’s got a few software hotshots that already have experience with the platform so that isn’t a worry. The big deal for her is moving from a relatively easy 64 lead QFP to the much more serious 457 ball .5mm pitch BGA.

She’s used a few small 1mm pitch BGAs before, but never anything close to this. It wasn’t just the package that got to her. She just about said “no” after reading about the 500MHz core speed for the new chip. Fortunately though, the i.MX has a rather large frequency pre-multiplier allowing for a rather sedate 32KHz reference clock to generate the screaming 500MHz+ internal clock. Still, there will be plenty of video speed and other high frequency signals running around. It will be a challenge.

There are some pretty intimidating parts here. A year ago, Jennifer wouldn’t have seen this challenge as anything but fun. The company had three layout specialists with experience on all kinds of high-speed designs. Two of those guys were laid off this spring and the last guy is dedicated to higher priority projects. It would have been nice if she could have gotten some formal training with the layout half of the CAD package but it’s all on-the-job training at this point.

Back to Mark Rules and the mini control board again. He's using Eagle CAD which comes with good and not so good. The good is that Eagle is pretty powerful, yet inexpensive and easy to learn and use. The no so good is that the parts libraries tend to not have the newest parts or packages. The means before getting started with the schematic, Mark had to create the libraries.

He found a PIC18 with a QFN28 package variant, but the package didn't quite match the manufacture recommendations or Screaming Circuits' guidelines. It was a decent start but needed adjustments on the solder paste layer and the solder mask layer.

He started with a similar DFN8 packaged regulator, but in the end found it easier to start from a blank slate. The DFN10 motor driver used the regulator part as a base but required a lot of work to get the copper aligned, the mask in the right place and the solder paste set up. The center pad now has a segmented solder paste patter with room for two rows of three thermal vias. He needs to find a board shop that can drill and fill 8 mil vias. The FlipChip package was fairly easy, but again, will require a board house that can really register the solder mask.

When we last left Mark Rules, he had made most of the component decisions for his miniature microcontroller and motor driver board. He still hadn't made a final call on the exact PIC processor. All of the options fit the same 6x6mm footprint, so there won't be any size changes regardless. Connectors looked to be the real problem.

The best he's found so far is a Digi-Key part number 609-1847-1-ND (x3), 609-1851-1-ND and 609-1854-1-ND. [Screaming Circuits will gladly accept Digi-Key part numbers in a Bill of Materials]. These are all larger then desired, totaling just over 200 square mm of board space, but it may not be possible to go smaller. That'll probably force all the connectors on one side and most of the passives on the other side with the IC's. We'll look at a couple of other suppliers just for kicks, but most likely, that will be it.

This job is mostly a layout challenge, since it's based on an existing design. There will be a few changes but not many. Still, there really isn't any safety in "just a layout." With big thru-hole parts, layout tends to not be all that important. However, when you start moving into higher speed and smaller geometries, layout becomes very important.

This isn't a high-speed design, but there are critical layout considerations. QFNs require special care. Check out our QFN layout guide. The guide will take you through the techniques required to create a solid, reliable design with QFN packaged parts. We also have our LED markation guide to promote more accurate assembly.

Speaking of LEDs, Mark ran into something else he hadn't thought of. In his prior design, he just used 1/8 Watt thru-hole resistors for the LEDs without giving thought to power requirements. His first thought here was to just use 0201 parts everywhere. That was until someone suggested he actually calculate out the power dissipation. Doing so brought the LED current-limit resistors up to 0603 size 1/10 Watt. Looking at the other resistors, he even had to move a few up to 0805 1/8 Watt.

Mark works for a small company that designs and markets microcontroller development boards and motor drivers. The products tend to be used in small robotics projects, mostly in the education and hobby markets. They are starting to see more and more commercial business though, which is driving changes in product requirements.

Mark's new project requires that he take a current design controller board and make it as small as is possible with as few circuit changes as possible. The current board is 2” x 2.5” and uses a PIC 8-bit processor. It’s a pretty simple design that can control a pair of motor drivers and accept a variety of I/O connections.

The new version will combine the dual motor drivers on the controller board and still have almost the same I/O capabilities. The PIC18F2321, in a 6x6mm QFN package, looks to be the smallest Microchip processor that supports dual hardware PWM. He considered a 4x4mm QFN PIC16F690 using software PWM, but didn't like the performance hit.

For 5V regulation, he found the Microchip MCP1726 in a 3x3mm DFN package. The RS232 driver comes from ST micro, ST3243, in a 2.4x4mm FlipChip BGA. The dual motor driver is a pretty impressive 3x3mm A3901 from Allegro. That’s 64 square millimeters of chip space using QFN and BGA packages instead of 266 square millimeters for the processor alone in the old design.

The passive components come out to 4x 0201 parts, 4x 0402, 3x 0603, 3x 0805 and 2x 3216-18 capacitors. All of those should fit on the back of the board with enough room left over for a small TVS that may or may not be needed to suppress EMI from the motors. A few LED status indicators should also fit nicely on the top of the board.